A new dynamic model involving upward migration of fluids from below the brittle crust

Stefánsson and Halldórsson (1988) proposed that there was interplay between local intrusions of fluids from below the brittle crust in the South Iceland Seismic Zone (SISZ) and strain buildup by plate motion and release in earthquakes. This was called the ‘‘dual-mechanism model’’ (described briefly in Chapter 3). This made a lot of sense for Iceland with its volcanism. However, in volcanic rift zones earthquakes are frequent, but small, rarely larger than around magnitude 5. Dyke intrusions there result in land deformation, volcanic eruptions, and areas of fumarolic high-temperature activity, which quickly and effectively release strains at seismogenic depths in the crust and do not lead to large earthquakes. This was in stark contrast with the non-volcanic transverse zones of Iceland where day-to-day seismicity is low but large earthquakes occur at long intervals. The dual-mechanism model could explain many observed seismic patterns in the transverse zones but it was not well constrained physically. It was revived at a meeting of the PRENLAB group in 1999 (Stefánsson, 1999) as it was considered to be a more robust physical model.